1. Field of the Invention
The present invention relates to a driving device for an ultrasonic wave motor for driving a movable member by means of a travelling vibration wave generated in an elastic member by a piezoelectric member.
2. Related Background Art
Conventionally there have been proposed various types of driving devices for the ultrasonic wave motor.
At first the structure of an ultrasonic wave motor will be briefly explained with reference to the attached drawings. FIG. 1 is a cross-sectional view of an ultrasonic wave motor, in which a rotor (movable member) is composed of a main rotor member 100-1 and a sliding member 100-2 which are mutually adhered. Also a stator is composed of an elastic member 100-3 and a piezoelectric member 100-4 which are mutually adhered. Said rotor and stator are maintained in mutual pressure contact to constitute said ultrasonic vibration motor.
FIG. 2 is a view showing the arrangement of electrodes in said piezoelectric member 100-4. input electrodes 100-4a, 100-4b receive AC drive voltages of a mutual phase difference of .pi./2. A common electrode 100-4c is grounded. A monitor electrode 100-4d does not contribute to the oscillation of the elastic member but provides an AC output voltage corresponding to the vibration state of the stator.
Thus the ultrasonic wave motor consists of the main rotor member, sliding member, elastic member and piezoelectric member explained above. The structure and function of such motor will not be explained in detail as they are already known for example from the U.S. Pat. No. 4,510,411. In brief, drive voltages are supplied to said electrodes 100-4a, 100-4b of the piezoelectric member to oscillate said member, thereby generating a travelling vibration wave in the elastic member, and driving said rotor maintained in pressurized contact with said stator by pressurizing means not shown.
A drive control device for such ultrasonic wave motor is already disclosed for example in the Japanese Laid-open Patent Appln. Nos. 59-204477 61-251490. These drive control devices are either (1) to control the frequency of the drive voltage signals according to the voltage obtained from the monitor electrode 100-4d, or (2) to control the frequency of the drive voltage signals according to the phase difference between the wave form of the drive voltage signals applied to the piezo-electric member 100-4 and that of the voltage signal obtained from the monitor electrode 100-4d.
In addition, for driving such ultrasonic wave motor, there have been proposed various methods by varying the drive voltage or by varying the drive voltage and the drive frequency, as disclosed for example in the Japanese Laid-open Patent Appln. Nos. 61-124275 and 62-19276.
In the drive control of the ultrasonic wave motor, it is known that the drive speed of the motor becomes unstable or the motor generates abnormal noises if the frequency of the drive voltages applied to the drive electrodes, namely the drive frequency, is selected close to the resonance frequency specific to the ultrasonic wave motor. It is also known that the motor loses the drive speed rapidly and enters a very unstable operation state if the drive frequency is selected lower than said resonance frequency.
FIG. 3 shows the relationship between the drive frequency and the drive speed of the ultrasonic wave motor, in which F1 indicates the resonance frequency specific to the ultrasonic wave motor. By dividing the entire frequency range into regions a and b by a drive frequency F3 slightly higher than said resonance frequency F1, the drive frequency region a suffers the above-mentioned abnormalities in the drive, while the drive frequency region b from said frequency F3 to a higher frequency F2 where the drive speed approaches to zero provides stable drive control. The conventional drive control devices for the ultrasonic wave motor effects the drive control within such frequency region b.
However the present applicant has experimentally confirmed that the following drawbacks are encountered even when the ultrasonic wave motor is driven with a frequency within said frequency region b allowing stable drive control:
(1) When the drive voltage is increased beyond a certain value (hereinafter called upper limit), the motor shows unstable drive speed or generates abnormal noises. This is presumably due to an undesirable relationship between the stator and the rotor maintained in pressure contact by the pressurizing means, induced when the amplitude of the travelling vibration wave exceeds a certain value;
(2) On the other hand, when the drive voltage is decreased beyond a certain value (hereinafter called lower limit), the operation of the ultrasonic wave motor becomes unsmooth and the motor may eventually stop. This phenomenon is presumably due to a fact that the oscillating force generated by the piezoelectric member cannot overcome the pressurizing force of the pressurizing means, due to the influence of the load, in the contact portions of the stator and the rotor, when the amplitude of the travelling vibration wave becomes smaller than a certain value.
Thus, in the speed control of the ultrasonic wave motor, the drive frequency may become positioned outside the region b providing stable drive because of the above-mentioned drawbacks it the drive voltage exceeds the upper limit or decreases beyond the lower limit. Thus the drive frequency enters the region a involving the above-mentioned unstable drive state, thus rendering the function of the ultrasonic wave motor unstable.
In addition, in case the drive voltage is given to the drive electrode of the motor from a power amplifier through a coil or a transformer provided therebetween, a resonance is generated between the inductance of such coil or transformer and the capacitance of the piezoelectric member, whereby the drive voltage on the drive electrode may fluctuate depending on the drive frequency even if the output voltage of the power amplifier is maintained constant. As a result, even if the drive frequency is selected at the middle of the region b, the drive frequency may be shifted from the stable region b due to fluctuation in the drive voltage, whereby the function of the ultrasonic wave motor may become unstable due to such unstable drive frequency.
A first aspect of the present invention, to be explained later, is to control the magnitude of the drive voltage between the upper and lower limits thereby driving the ultrasonic wave motor in stable manner.
However, the conventional drive devices for the ultrasonic wave motor are still associated with a drawback that they cannot constantly provide a high driving efficiency even when a stable drive state is obtained, as will be explained in the following with reference to the attached drawings. FIG. 4 shows the drive current, drive speed and drive efficiency as a function of drive frequency, under a drive voltage of 25 VRMS and a load torque of 900 g.cm, measured by the present inventor. From a lower frequency side, the drive current reaches a maximum at a drive frequency F0, then decreases to reach a minimum at a drive frequency F4, and gradually increases again at the higher frequency side. On the other hand, the drive speed is highest at a frequency F1, and decreases both at the higher or lower drive frequency sides. The frequency region usually used for driving the ultrasonic wave motor is at the higher frequency side of said frequency F1. The drive efficiency is highest in the vicinity of the drive frequency F4 where the drive current is lowest. As will be understood from these facts, the drive efficiency becomes highest in the vicinity of the frequency F4 within a frequency region higher than the frequency F1, and the conventional drive devices are unable to constantly provide the high drive efficiency since they are not designed to control the drive frequency at the highest efficiency.
Therefore, a second aspect of the present invention, to be explained later, is to provide a driving device for the ultrasonic wave motor, capable of improving the drive efficiency thereof.